Spectrum sharing is thought of as one of the most viable ways of improving the amount of spectrum available to wireless networks and other radio devices for conducting wireless communications. An exemplary spectrum sharing technique involves use of television white spaces under regulations set forth by an appropriate regulatory agency. An exemplary regulatory agency that regulates the use of wireless spectrum is the U.S. Federal Communications Commission (FCC). Other countries may have similar regulatory entities.
In the U.S., for example, the FCC has eliminated analog television (TV) broadcasts in favor of digital TV broadcasts. This has freed spectrum channels for use by unlicensed radio systems to offer various services, such as mobile communications and Internet access. In this context, the freed spectrum is commonly referred to as TV white space (or TVWS) but other types of white spaces are possible. In the case of TV white space, the white space is comprised of unused spectrum that is interleaved with spectrum used by incumbent radio devices in the channel 2 to channel 51 range (corresponding to 54 MHz to 698 MHz). Exemplary incumbent radio devices for TV white space include television broadcasters and other priority users of television channels.
Under FCC regulations, radio devices that use TVWS must register with a central database server (also referred to as a spectrum management server) and receive a channel list (also referred to as a channel map) of available channels for which the radio device may use in a shared environment with other TV band devices (TVBDs) while minimizing the possibility of creating undesirable interference to incumbent radio systems. The channel list that is generated for a radio device is generated by the central database server based on the location of the radio device. In this manner, the operation of incumbent radio devices having protected areas in which the radio device is located may be taken into account when determining channel availability.
Also, regulators and industry groups have proposed the use of geo-location database technology to control or manage spectrum access for radios in other situations. For example, use of geo-location database technology has been proposed for the 5 GHz UNII bands and for the 3.550 GHz to 3.650 GHz bands in which the U.S. government and military are incumbent users.
Outside the U.S., use of geo-location database technology has been proposed for TVWS bands in a number of countries. In the European Union (EU), TVWS sharing is often referred to as authorized shared access (ASA) and/or licensed shared access (LSA). Exemplary regulations for ASA/LSA are outlined in European Telecommunications Standards Institute (ETSI) EN 301 598. Other standards bodies have also proposed mechanisms for supporting spectrum sharing, such as the Internet Engineering Task Force's (IETF) Protocol to Access White Space (PAWS).
In cognitive radio ecosystems, such as the above-mentioned TV white space environment, it is common for a first radio device (e.g., a first TVBD) to establish wireless connectivity with a second radio device (e.g., a second TVBD). Under controlling regulations, the connectivity typically must be established without causing interference to primary, incumbent or other concurrent radio devices (collectively referred to as incumbent users).
An exemplary radio device involved in this situation may be a fixed-location TVDB device that is connected to the Internet over a medium that does not cause impermissible interference to another radio device. The medium over which Internet connectivity is made may be, for example, a terrestrial connection or a cellular connection. As will become clear from the following, this radio device may be considered a hub radio device due to its relationship with another radio device, which may be considered a spoke radio device.
The hub radio device may acquire a channel list from a geo-location database (also referred to a spectrum management server). The channel list authorizes the hub radio device to transmit on specified channels, at a specified location, at a maximum power, and for a specified period of time.
An issue arises when the spoke radio device (another TVBD in the example), which does not have independent access to the Internet, attempts to acquire a channel list from the geo-location database using a radio link established with the hub radio device as a pathway for Internet access. The two radio devices are considered hub and spoke radio devices due to their relationship with one another relative to the Internet connection.
Since the spoke radio device is in a different location than the hub radio device, an authorized channel list for the spoke radio device may not include an authorization for the same channel(s) used by the hub radio device. The channels used by the hub radio device may not be authorized for use by TVBDs, including the spoke radio device, in the location of the spoke radio device due to presence of an incumbent user with a protected boundary area that encompasses the location of the spoke radio device, but not the location of the hub radio device.
This is a dilemma. But, at least in the U.S., the Federal Communications Commission (FCC) considers this a special situation and allows limited use of the operational channel of the hub radio device by the spoke radio device under the following regulation set forth in 47 C.F.R. §15.711(e):                Fixed devices without a direct connection to the Internet. If a fixed TVBD does not have a direct connection to the Internet and has not yet been initialized and registered with the TV bands database consistent with §15.713, but can receive the transmissions of another fixed TVBD, the fixed TVBD needing initialization may transmit to that other fixed TVBD on either a channel that the other TVBD has transmitted on or on a channel which the other TVBD indicates is available for use to access the database to register its location and receive a list of channels that are available for it to use. Subsequently, the newly registered TVBD must only use the television channels that the database indicates are available for it to use. A fixed device may not obtain lists of available channels from another fixed device as provided by a TV bands database for such other device, i.e., a fixed device may not simply operate on the list of available channels provided by a TV bands database for another fixed device with which it communicates but must contact a database to obtain a list of available channels on which it may operate.        
A consequence of this pragmatic methodology is the creation of a transient condition that might cause brief unintended interference to operations of incumbent users.
Other approaches may result in comparatively less transient interference in this situation. For example, certain regulations do not allow the flexibility of temporary communications that exceed certain transmit power thresholds. Unfortunately, these constraints have consequences that may promote unreliable or artificially hindered communications. As an example, European regulators have specified a situation in which a fixed device (e.g., the hub in the foregoing example) with a terrestrial connection to a database/server may acquire a “generic” channel authorization list and a “specific” channel authentication list. The specific channel authorization list allows normal use of the channel(s) in the list with other devices that also have specific authorization to use the channel(s). The “generic” channel authorization list allows communications with devices that do not have specific authorization, but these communications are more constrained.
The generic list of channels is the same as the specific list of channels, but includes a transmit power limit for each channel. The generic transmit power limit is the lowest authorized transmit power at any location in the hub's coverage area. When the spoke is operating in accordance with a generic channel authorization, regardless of its location in the coverage area of the hub, the spoke may not transmit with a power above the generic power limit for the relevant channel. The transmit power limit values may differ on a channel-by-channel basis.
More specifically, the area that includes the coverage area of the hub radio device is broken up into “pixels” where each pixel is a square unit area or some other predefined shape and area. The generic transmit power limit for each channel is determined by comparing the maximum allowable transmit power for every “pixel” overlapping with the coverage area and selecting the lowest value for the channel. This “lowest value” represents the maximum allowable transmit power for the channel that may be used by another non-specifically authorized device during communications with the hub (e.g., to use the hub as a pathway or proxy to obtain a channel list). To make other radio devices aware of the generic channel availability, the hub radio device broadcasts a beacon with a list of channels and an allowable transmit power for each channel in the list.
An exemplary channel list with generic operating parameters for the entirety of the hub's coverage area may be specified in a simple list format or in a table format with channel and transmit power entries (CH, TX POWER). An example is set forth in table 1.
TABLE 1ChannelTransmit PowerCH 10 dBmCH 212 dBm ...... CH 133 dBm
Under this approach, the spoke radio device may use the generic operational parameters specified in the beacon to communicate with the hub, including the generic transmit power as a maximum transmit power. By way of these communications and using the hub radio device as a communications pathway or proxy, the spoke radio device may request specific operational parameters (e.g., an authorized channel list) for the geographic location of the spoke radio device from the server hosting the geo-location database functions.
But, as shown in FIG. 1, it is likely that the specific operational parameters will be more favorable (i.e., allow higher transmit power) and enable a more reliable communications link than the generic parameters. FIG. 1 shows an exemplary hub radio device 10 and a coverage area 12 of the hub radio device 10. In the illustrated example, the coverage area 12 has a maximum range of about 10 miles (about 16 kilometers) from the location of the hub radio device 10. The geographic area that includes the coverage area 12 is divided into square-shaped pixels. In each pixel, a representative maximum allowable transmit power in dBm for the pixel is shown for a single channel. Also shown are two exemplary spoke radio devices 14a and 14b at different locations in the coverage area 12 of the hub radio device 10. In the illustrated example, the hub radio device 10 may be authorized to transmit at 30 dBm at its specific location. If it had specific channel authorization to use the channel of the illustrated example, the spoke radio device 14a also would be able to transmit at 30 dBm. But the lowest allowable transmit power in the exemplary coverage area 12, which occurs at the location of the spoke radio device 14b, is only 0 dBm. This lowest allowable transmit power represents the generic transmit power limit for the channel for the entire coverage area 12. Therefore, without specific authorization, the spoke radio device 14a in this example may communicate with the hub radio device 10 using a maximum transmit power of 0 dBm.
In this scenario, is that it is highly unlikely that the hub radio device 10 will receive transmitted data from the spoke radio devices 14a, 14b (e.g., a specific operating parameter request) since the transmit power of the spoke radio devices 14a, 14b is limited to be much less than the transmit power of the hub radio device 10. Signal reception will still be a problem even if the data rate is drastically reduced to increase link budget. Moreover, reducing the data rate further complicates the situation by requiring the hub radio device 10 to listen at varying data rates.
Another problem occurs if the hub radio device 10 and the spoke radio device 14a or 14b are able to initially establish and achieve reliable communications that enable the exchange and confirmation of specific operating parameters with higher allowable transmit power. Establishment of such a link might occur during quiet conditions, such as at night. But, from time to time, the hub radio device 10 may reacquire specific operating parameters due to a reboot or expiration of earlier specific operating parameters. In this case, further communications over the original link between the hub radio device 10 and the spoke radio device 14 may not be possible, thereby artificially inhibiting communications.
Still other issues arise in asymmetrical communication links. The hub radio device 10 may acquire a channel list with specific operating parameters for its specific location and acquire a channel list with generic operating parameters to be offered to spoke radio devices 14 that surround the hub radio device 10. Ideally, some channels in both the specific and generic lists will have similar and favorable operating parameters affording the opportunity to enable robust bi-directional communications between the hub radio device 10 and spoke radio devices 14. However, asymmetry between hub transmit power and spoke transmit power will often exist in this scenario. Link asymmetry can be tolerable and even normal in some systems in which sophisticated base stations utilize large, high gain antennas, extremely sensitive receivers and high power transmitters to accommodate low power, low cost mobile devices. An example of this type of system is cellular telephony where channel pairs are used for an uplink and a downlink in a frequency division multiplex (FDD) protocol. However, asymmetric link architecture is not the ideal scenario for low cost and less sophisticated radio systems, such as most TVBDs.
As indicated, to determine generic operating parameters, a geography-by-pixel and channel-based scheme may be used. An exemplary scheme of this nature that is used by cognitive radios is described in ETSI 301 598. This approach relies on a system in which the maximum allowable transmit power (per channel) is calculated for a grid (by pixel, e.g. 100 m×100 m) to ensure that devices do not cause interference with other radio devices or services. The maximum transmit power and other operational parameters for a location are determined based on radio propagation models and parameters, such as out of band emissions, antenna gain, antenna height, terrain, clutter, radio performance and other parameters. Table 2 contains a channel list with specific operating parameters and generic operating parameters that are generated for a hypothetical hub radio device under the approach described in ETSI 301 598.
TABLE 2Hub's Specific OperatingSpoke's Generic OperatingParameter - MaximumParameter - MaximumChannelAllowable TX Power (dBm)Allowable TX Power (dBm)CH 115−10CH 21910CH 32−38CH 413−21CH 51715CH 612−17CH 725−18CH 87−23CH 919−24
In this example, choosing the best channel pair for bi-directional communications between the hub radio device 10 and the spoke radio device 14 is straightforward. The hub radio device 10 is authorized to operate at +17 dBm on channel 5 while the spoke radio device 14 (with undetermined location in the hub's coverage area 12) is concurrently allowed to operate at +15 dBm on the same channel. This enables both radios to operate almost symmetrically at relatively high power. However, if this choice did not exist, the selection is not as clear. Note, for example, that channel 7 permits the hub radio device 10 to operate at a relatively high power of +25 dBm, but the permitted transmit power for the spoke radio device 14 on channel 7 is −18 dBm. Such a low permitted transmit power for the spoke radio device 14 would lead to very poor performance and may not support communications. It may then be concluded that channel 2 is the next best choice where the specific operating parameter for the hub radio device 10 is 19 dBm and the generic operating parameter for the spoke radio device 14 is 10 dBm. But this is a 9 dB difference at lower transmit power for the spoke radio device 14 than allowed on channel 5.
The hub's coverage area 12, in terms of size and range, is directly proportional to transmit power. A good link budget translates to increased range, which has the effect of including more pixels in the generic parameter determination process. The inclusion of more pixels in the coverage area 12 increases the chance that the generic transmit power for a given channel (i.e., the worst case, or lowest, allowable transmit power for all pixels in the hub's coverage area), will be lower than for a smaller coverage area. As a result, asymmetric links between the hub radio device 12 and spoke radio devices 14 may result from the use of high specific transmit power by the hub and generic transmit power by the spoke(s).